Hillary Clinton’s campaign for the presidency just got harder with the release of the State Department inspector general’s finding that “significant security risks” were posed by her decision to use a private email server for personal and official business while she was secretary of state.

So far, no security breaches have been reported; a separate F.B.I. investigation is looking into that. But above and beyond security questions, the inspector general’s report is certain to fuel doubts about Mrs. Clinton’s trustworthiness, lately measured as a significant problem for her in public polls.

Across the years of the Clintons’ ascendancy, the public has seen that Mrs. Clinton can be fiercely protective of her role and prerogatives — at times grudging in admitting error and, during Bill Clinton’s presidency, blaming a “vast right-wing conspiracy” for allegations against her and her husband that began early in his tenure and continued on through the impeachment scandal. (The right wing was definitely on his case, but hardly alone in its doubts about Mr. Clinton’s personal conduct.)

This defensive posture seems at play in the email controversy, as well as her refusal, for that matter, to release the lucrative speeches she made to Wall Street audiences. The reflex she is revealing again now — to hunker down when challenged — is likely to make her seem less personable to many voters, and it will surely inflame critics’ charges of an underlying arrogance.

Donald Trump, her Republican rival, will be merciless in swinging the inspector general’s report like a cudgel. Accordingly, Mrs. Clinton now faces a measurably greater challenge in proving that she is the well-qualified politician her supporters know her to be, based on her varied career as a senator, secretary of state and first lady deeply involved in public life. This is a challenge to be faced not with a contrived campaign makeover, but with a far greater investment of candor before the public.

When Republicans first questioned the propriety of using her own home-based server over a year ago, Mrs. Clinton sought to finesse the matter as partisan flak. Under pressure, she eventually apologized for a “mistake,” while insisting she had done nothing wrong and would cooperate fully with investigators. But she did not honor that promise, according to the report, which noted that she declined to be interviewed by the inspector general, Steve Linick, or his staff.

When State Department staff members questioned her use of a nongovernmental email address in 2010, the report said, they were instructed by superiors “never to speak of the secretary’s personal email system again.” The sharply critical report found that, contrary to her earlier insistence that the practice was “allowed,” Mrs. Clinton had not sought permission to use the server, and that permission would have been denied under the department’s evolving policy to better protect Internet communications.

What follows next is certain to be a grueling campaign slog through details and allegations that voters will be hard pressed to track and, indeed, may soon tire of. Even now, it seems a stretch to say that Mrs. Clinton’s email mishaps should disqualify her for the White House, particularly considering the alternative of Mr. Trump with his manifold evasions — not least his refusal to release tax returns that could shed light on his claims to great wealth, his charitable contributions and other deductions and possible conflicts of interest.

But the nation should not be judging leadership as a measure of who is less untrustworthy. Mrs. Clinton has to answer questions about the report thoroughly and candidly. That is her best path back to the larger task of campaigning for the presidency.